1. Field of the Invention
This invention relates to telephone systems and more particularly to intercommunicating systems which use existing single-line quadwire equipment for both intercom and normal telephone functions.
2. Description of the Prior Art
Many households presently have several telephone extensions distributed throughout the house connected to a single telephone line. The extension telephones enable a party to answer or initiate a call from any of the telephone locations. The convenience of an extension telephone is enhanced by the use of an intercom system which enables the answering party to call another person to the phone by "paging" that person over the intercom rather than by shouting aloud for that person.
Some home telephone adjuncts or intercom systems also provide a capability to allow the answering party to return the handset to the on-hook position and then pick up the call at another telephone location without dropping the call. Such a "hold" feature together with a "paging" capability are part of the prior intercom art. Additionally, some prior intercom art allows for signalling a busy intercom connection that an outside call is ringing (call waiting feature).
An intercom system to be used with a single-line installation must be economically justifiable, should not preclude use of the existing nonkey telephone subsets in the normal way, and also use the four-conductor telephone cable that is normally prewired in homes.
The problem with the prior art intercom systems which are used in conjunction with an existing single-line telephone system is that some intercoms require more than the standard four-conductor telephone cable to provide an intercom function. However installation of special wiring requires special skills and it is difficult and expensive to accomplish in existing structures.
The hold, paging, and call waiting features also are available in the commercial environment in the form of key telephone systems. These key systems, however, are not always practical or economical for single-line installations since they require key telephones, line circuits and additional wiring to provide the required functions. Such systems are not cost effective for less than 10 sets and they do not normally interface with existing non-key type subsets.
In the prior art U.S. Pat. No. 4,196,317 to Bartelink describes a relatively simple system in which manual switches are used to switch a given subset between a central office line and an intercom line. Intercom call signalling is accomplished by transmitting an audio signal on the intercom line, which must be interpreted by the users.
Use of central control electronics puts a financial burden on small system purchasers, and thus distributed control is preferred over centrally controlled systems. The following prior art patents disclose systems with central control electronics. U.S. Pat. No. 4,088,846 to McEowen discloses a system comprising subsets with adjuncts interconnected by two lines and a common control circuit. One line is used for voice control and supervisory signals. The second line is used to control and power the subset adjuncts.
U.S. Pat. No. 4,079,212 to Sasai shows another example of a system in which subsets are connected to a common control (or main) equipment by a first line for voice signals and a second line for control signals.
Finally, U.S. Pat. No. 4,100,375 to Noller shows a system in which each subset and device is wired in parallel to a control circuit.